1. Field of the Invention
The present invention relates to a drive control device for a servomotor used as a drive source for machine tools, industrial machinery, robots, and other equipment controlled by a numerical control device (NC device).
2. Description of the Related Art
In machine tools, synchronous control is sometimes performed whereby an object is driven by a plurality of motors. Twisting may occur in a workpiece by disturbance that is external to the machining process when a large workpiece is driven by means of a single servomotor, as is the case with the C-axis of a crank grinder, for example. Such twisting of the workpiece affects machining accuracy. Workpiece twisting is reduced by providing two servomotors to the workpiece and synchronously controlling both servomotors so as to maintain synchronization in response to the twisting of the workpiece.
With this type of synchronization, a servomotor is provided to each of two shafts that are connected to the workpiece, and the servomotors are controlled with their respective servo circuit. Each servo circuit has a position control unit, velocity control unit, and electric current control unit, and receives the same position command from a numerical controller. The two servomotors, in order to correct synchronization misalignment, compute the correction amount with the aid of feedback values of the positions, and make corrections by which this correction amount is added to the position command from one of the servo circuits. An example of such prior art is disclosed in Japanese Patent Application Laid-open No. H11-305839, for example.
In synchronization control for driving one object with a plurality of motors, each motor operates by receiving the same position command from a host control device, and the position of each motor is controlled such that the position feedback values from the respective position detectors are in agreement with the position commands. In this type of synchronization control, the accuracy of the position detectors may be inadequate when the motors move in accordance with the position commands, or the actual end-point position may occasionally shift away from the instructed position when the machine is affected by thermal expansion. The reference position as such is also indeterminate because the scale itself, which is the positional reference, occasionally becomes displaced due to thermal expansion, for example.
When the rigidity between the motors is high in such a case, drawbacks arise in that a phenomenon occurs whereby the motors pull against each other so that stress is generated between the motors; the motors and amplifiers become heated; and machining accuracy decreases.